Currently, nitric acid is produced industrially via the catalytic oxidation of ammonia, over a platinum or platinum alloy-based gauze catalyst. This process, known as the Ostwald process, has essentially remained unchanged, since its inception in the first decades of the twentieth century. Ostwalds's patent was dated 1902 and when combined with Haber's development of synthesising ammonia, in 1908, the basis for the commercial production of nitric acid, which we use today, was in place.
The combustion of ammonia is carried out over a platinum-based metal or alloy catalyst in the form of a gauze or mesh or net. A number of gauzes are installed together, and they constitute the gauze pack. The upper-most gauzes have compositions optimised for the combustion of ammonia, and are referred to as the combustion gauzes. Gauzes with other compositions may be located below the combustion gauzes, and these may have other roles, as described below. The whole stack of gauzes is referred to as the gauze pack. The gauzes are produced either by weaving or knitting.
The operating temperatures of the plants are typically 850 to 930° C. and the range of pressures is from 15 bara to 1 bara. Typically, the combustion gauzes are installed in the plant for between six months and two years, depending on the plant operating. conditions. Plants operating at high pressures typically have shorter campaigns than low-pressure plants.
The duration of the campaign is governed by a loss in the selectivity of the catalyst, towards the desired nitric oxide product, through the increased formation of unwanted nitrogen and nitrous oxide by-products. The loss of selectivity is related to a number of phenomena. During combustion, platinum is lost through the formation of PtO2 vapour. Some of the platinum may be recovered by the installation of palladium metal-based gauzes, directly below the platinum-based combustion gauzes. The PtO2 vapour alloys with the palladium, therefore, platinum is retained in the catalytically active zone. However, due to the depletion of platinum in the upper combustion zone of the gauze pack, not all of the ammonia is immediately combusted. If the ammonia is combusted in the palladium gauze region, the selectivity towards nitric oxide is reduced, and secondly, if ammonia and nitric oxide coexist in the vapour phase for a period of time, nitric oxide is reduced by ammonia, through a homogeneous reaction. This leads to both nitric oxide and ammonia losses. A final mechanism for loss of selectivity is related to the fact the platinum is lost from the combustion gauzes at a higher rate than the other alloying elements (typically rhodium). This leads to rhodium enrichment of the gauze surface which leads to selectivity loss.
Over the last sixty years, many attempts have been made to replace the expensive platinum-based combustion catalyst with lower cost catalysts, based for example on metal oxides. To date, the only commercially available oxide-based catalyst for ammonia combustion was developed by Incitec Ltd (Australia). This is based on a cobalt oxide phase. However, in terms of its selectivity of combustion of ammonia to the desired nitric oxide product, its performance is inferior to that of platinum-based systems. The cobalt oxide based systems have shown selectivity levels towards nitric oxide and nitrogen dioxide of circa 90%, in commercial units, compared to the 94 to 98% achieved with platinum-based catalysts.
Another Norwegian Patent application NO20074325 relates to another oxide catalyst composition of the formula A3−xBxO9−y, wherein A and B independent of each other are selected from the group Mn, Co, Cr, Fe and Al, x is between 0 and 3 and y is between 0 and 5. This catalyst composition has high activity towards nitric oxide and provides high selectivity.